The present invention relates to an electric hand having a pair of workpiece-grasping jaw members which are opened and closed by an electric motor, and more particularly, to an electric hand with a torque limiter in which no impact is applied to a motor drive shaft when the jaw members are stopped at an opening or closing end or a workpiece-grasping position.
In generally, in a hand for grasping a workpiece between a pair of jaw members which can be opened and closed, the jaw members are opened and closed by an air cylinder.
However, a conventional hand of such a structure requires an air piping with respect to the air cylinder and additional devices such as a direction-switching valve for controlling the supply and discharge of compressed air to the piping, and the number of parts constituting the hand itself is increased. For this reason, it is difficult to make the hand compact, and to make the entire apparatus including the additional devices for operating the hand compact.
To solve such a problem, the present inventors proposed an electric hand for opening and closing a pair of workpiece-grasping jaw members using an electric motor (Japanese Patent Application No. 11-66528). In this electric hand, a conversion mechanism using a cam is provided between a drive shaft of the electric motor and the jaw members for converting a normal-reverse rotational motion of the drive shaft into an opening and closing motion of the jaw members.
In such an electric hand, when a workpiece-grasping force by the jaw members is increased, a motor with a speed reducer having a plurality of gears is used as an electric motor. When such a motor with gears is used, however, since an inertial force due to a rotor of the motor is applied when the jaw members stop at the opening and closing end or the workpiece-grasping position and the gears of the speed reducer are damaged by the impact, the motor must be rotated as slow as possible.
In an electric hand for opening and closing a workpiece-grasping jaw members using a motor, it is an technical object of the present invention to absorb an inertial force applied to a motor drive shaft when the jaw members stop at an opening or closing end or a workpiece-grasping position, thereby preventing a motor part such as a gear from being damaged by the impact.
To achieve the above object, the present invention provides an electric hand comprising: a pair of jaw members which can be opened and closed for grasping a workpiece; an electric motor having a rotating drive shaft; a converter mechanism provided between the drive shaft and the jaw members for converting a normal or reverse rotation motion of the drive shaft into an opening and closing motion of the pair of jaw members, the converter mechanism having a cam member which is normally or reversely rotated by the drive shaft, a pair of cam grooves formed in the cam member, and drive pins respectively mounted to the jaw members and fitted to the pair of cam grooves, the drive pins are displaced in the cam groove by a normal or reverse rotation of the cam member in an opening or closing direction of the jaw members; and a torque limiter provided between the drive shaft of the electric motor and the cam member for limiting a torque of the drive shaft transmitted to the cam member.
The electric hand of the invention having the above structure, since a torque transmitted from the drive shaft of the electric motor to the cam member is limited by the torque limiter, it is possible to absorb an inertial force of a rotor of the motor applied to a motor drive shaft when the jaw members stop at an opening or closing end or a workpiece-grasping position, thereby reliably preventing a motor part such as a gear from being damaged by the impact.
According to a concrete embodiment of the invention, the torque limiter includes friction means for connecting the drive shaft and the cam member by a frictional force. The friction means includes a disc-like hub fixed to the drive shaft, a friction plate fixed to one of the hub and the cam member and abutted against the other in a friction state, and spring means for biasing the friction plate in a direction abutting against the hub or the cam member. More specifically, the cam member is of circular cup-like shape, the hub is coaxially fitted into the cam member, the friction plate is of an annular shape, an outer periphery of the friction plate is fixed to an annular mounting face of the cam member surrounding the hub, an inner periphery of the friction plate abuts against the hub, the spring means comprises a Belleville spring, the Belleville spring is interposed between a spring seat mounted to the drive shaft and the friction plate.